drm/amd/display: Filter out YCbCr420 timing if VSC SDP not supported

[Why]
Per DP specification, YCbCr420 shall use VSC SDP.

[How]
For YCbCr420 timings, fail DP mode timing validation
if DPCD caps do not indicate VSC SDP colorimetry
support.

Signed-off-by: George Shen <george.shen@amd.com>
Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Acked-by: Wayne Lin <Wayne.Lin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>